doteveryone explains

Uploading a photo: what data is sent?

You send the data you can see directly on the upload page, and some data you can’t.

Your BBC Weather Watchers upload also sends your login identity, so your photo can be linked to your profile.

On other sites, this extra data might be the date and time you make a post, or the clicks you took to arrive on a page.

When a form is submitted, some data you can see is submitted, along with some that you can't

Now learn about where your data is sent and how your device finds that location.